[SYCL] Rebasing the SYCL support branch on top of the Einge upstream master branch.

* Unifying all loadLocalTile from lhs and rhs to an extract_block function.
* Adding get_tensor operation which was missing in TensorContractionMapper.
* Adding the -D method missing from cmake for Disable_Skinny Contraction operation.
* Wrapping all the indices in TensorScanSycl into Scan parameter struct.
* Fixing typo in Device SYCL
* Unifying load to private register for tall/skinny no shared
* Unifying load to vector tile for tensor-vector/vector-tensor operation
* Removing all the LHS/RHS class for extracting data from global
* Removing Outputfunction from TensorContractionSkinnyNoshared.
* Combining the local memory version of tall/skinny and normal tensor contraction into one kernel.
* Combining the no-local memory version of tall/skinny and normal tensor contraction into one kernel.
* Combining General Tensor-Vector and VectorTensor contraction into one kernel.
* Making double buffering optional for Tensor contraction when local memory is version is used.
* Modifying benchmark to accept custom Reduction Sizes
* Disabling AVX optimization for SYCL backend on the host to allow SSE optimization to the host
* Adding Test for SYCL
* Modifying SYCL CMake

unsupported/doc/Overview.dox

@@ -11,6 +11,8 @@ Click on the \e Modules tab at the top of this page to get a list of all unsuppo
 
 Don't miss the <a href="../index.html">official Eigen documentation</a>.
 
+ \subpage SYCL_EIGEN "SYCL backend for Eigen"
+
 */
 
 /*
@@ -26,3 +28,4 @@ subject to be included in %Eigen in the future.
 /// \internal \brief Namespace containing low-level routines from the %Eigen library.
 namespace internal {}
 }
+